CA1038411A - Method for mining oil shales, tar sands and other minerals - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In the mining of subsurface friable mineral ore deposits, the invention provides a mine configuration and mining system wherein the mine has preferably three levels. The first operating level, is positioned sub-stantially at the base of the deposit and has an interconnecting system of tunnels within which, mining apparatus operates. This level further in cludes conveyors adapted to receive loose ore and transport same to ver-tical shafts or ore passes which interconnect adjacent operating levels.
The second operating level is primarily a collecting level, and is pos-itioned below the first, or mining level, to be parallel therewith. This second level has a tunnel system operatively associated with the vertical ore passes of the first level, and has conveyors which collect the loose ore and carry same to shafts for transport to a final, lower lever, from which the ore is removed from the mine. According to the invention, ac-tive mining is carried out by the known "Fixed Arch Shield" technique on only one level, the only other levels being ore collecting and haulage levels. There are no levels, or mine workings, above the first, mining level, only access and ventilation shafts. By utilizing this mining method and the preferred mine configuration, a complete ore deposit can be removed with minimal surface disturbance, and subsidence can be com-pensated for by distributing waste in a manner that is esthetically acceptable.

Description

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This invention relates to the mining of "oil shales" as found in Colorado, Wyoming, and Utah, U. S. A., and similar deposits of coal, oil shales, oil sands and other minerals found elsewhere.
The mining of oil shales appears to be the most imminent appli-cation of this method and, within the context of this application, mention is made of oil shales to the practical exclusion of other minerals. It should be understood that, wherever oil shales are mentioned, reference could be made to other minerals.
The term "oil shale" refers to sedimentary deposits of organic-rich argillaceous dolomites and marls. The organically derived component is a hydrocarbon called "kerogen", a mineral that when heated to a suff-iciently high temperature is converted to a form of crude petroleum.
Deposits of oil shale, occurring principally in the States of Colorado, Utah, and Wyoming, U. S. A., contain billions of barrels of crude oil . ~i reserves.

The U. S. Bureau of Mines estimates that 81 billion barrels , .
of liquid fuels will be required for consumption in the immediate future.
;;, This is more than was produced in the United States during the 100 years after oil was discovered in 1859. Oil shales are therefore a logical . ., : .
` 20 source of a substantial portion of this requirement.
While the oil shales constitute a resource, they cannot be utilized until some means is developed which will allow its exploitation - with safety and economy. The deposits of oil shale are enormous but they are also low-grade containing only a very small amount of oil per ton and this means that the cost of mining a ton of raw shale ore must be relative-ly low to make the work economically justifiable.
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Investigations to date have been of a pilot basis. These include the plant operations of Union Oil Co., the U. S. Bureau of Mines, the Colony Development Company joint venture, etc, but an economically viable method has not yet been demonstrated.

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There has been a voluminous amount of publication on the subject of mining oil shales. Reference of interest, to list only a few include:-~1) "Methods for Mining Deep, Thick Oil Shale Deposits," Whiting, J. M., United States Patent No. 3,588,175, dated June 28, 1971;

(2) Proposed for Oil Shale Development, Colorado, Utah and Wyoming, U. S. Department of the Interior, Washington, D. C., May, 1968;

(3) Allsman, P. T., "A Simultaneous Caving and Surface Restoration System For Oil Shale Mining, "Quarterly of the Colorado School of Mines, Golden, Colo. Vol. 63, No. 4, Oct. 1968, pp. 113-126, and:

(4) Cameron and Jones, Inc. "Comments and Suggestions on the Department of Interior Oil Shale Policy Statement of May, 1968" Synthetic Fuels Quarterly Report, Cameron and Jones, Inc., Denver, Colo.

Most of the attempts at mining oil shales have been by the con-ventional room-and-piller method in relatively thin strata. This is appli-';
cable to near-surface oil shale deposits which have a thickness from 20 to possibly 100 feet. Beyond a thickness of 100 feet, the "rooms" become too - high to be worked by normal room-and-piller methods, the method becomes very dangerous and nearly 50% of the oil shale must be left in pillars.

The oil shale deposits are very thick and removing 100 feet from any part of that thickness,while destroying the remainder for practical mining, is not acceptable.
It has been suggested that the thick oil shale strata could be mined successfully by open pit methods. While technically possible, the magnitude of the open pit and the enormous pile of tailings would be ecol-ogically unacceptable even though the open pit might eventually be filled ~-in and become esthetically acceptable.
A variation of block caving method has been sugges'ed for extracting deep thick strata of oil shale, ("Allsman, P. T. A Simultaneous Caving and Surface Restoration System For Oil Shale Mining", Ibid.~ The major obstacle to this appears to be the lack of control of caving.

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1~38411 Another possible mining method, referred to as "cut-and-fill" ~ ~-; has been suggested and forms the basis of the previously mentioned United States Patent Serial No. 3,588,175.
All of these methods have been considered and investigated and found to be either too ha~ardous or too expensive. In most cases they are both hazardous and expensive. Possibly of equal importance is that they have not been esthetically acceptable to the conservationists and ecolo-gists who are very jealous of the semi-mountainous area in which the de- `
posits occur.
Accordingly, the primary objective of the present invention are to:
(a) remove the oil shale at a cost that is commercially acceptable for production of crude oil;
(b) increase the safety of the miners excavating the oil shale, and;
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(c) conduct the excavation and disposal of waste in a manner which will be innocous, and barely discernable to anyone.
The present applicant, in Canadian Patent Application Serial Number 195,228, filed March 18, 1974, teaches what is referred to hereafter as the "Fixed Arch Shield" technique for mining oil shale, and tar sands.
Briefly, this technique provides a mining method that only requires active mining on a single level, at the base of an ore block to substantially remove 100% of the mineral block.
; This technique involves establishing a laterally extending underground mining face of perhaps 1,000 feet in width and from 10 to 12 feet in height, against which a laterally extending mining shield is pos-itioned, the shield serving to partially enclose mining machinery such as lateral shearing ploughs, rotary cutters or the like which are convention-al in underground coal mining, the cutter operating across the full width of the mining wall. The mined material, after being cut from the mining face, falls onto a lateral conveyor operating within the mining shield, -; ' '.~' ~' .
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' 10389~1 and is then conveyed from the face to collecting means, operating at the ends of the face for ultimate conveyance through a shaft to the ground surface.
The mining shield comprises a plurality of individual arch sections, each individually advanceable towards the mining wall as the mining wall recedes, thereby creating a void posteriorly of the mining shield, leaving the "back" unsupported and permitted to collapse. Nor~
mally, the back will not subside for several hours after the advance of the shield, due to the "semi-plastic" nature of the sand and the entrained viscous bitumen. Under normal mining operations, the back will collapse gradually, 20 to 40 feet behind the moving mining shield as it advances into the bituminous sand body.
The operation will proceed for an optimum distance of forward travel, and is then repeated, to operate on the collapsed material as before. It will be appreciated that the entire operation takes place on, ~; or adjacent the basement rock, the operation proceeding until the desired `
section of bituminous material has been mined out.
~ he cutting equipment is not manned, the cutter travelling between a pair of operators, one at each end of the assembled movable shield each such operator being positioned in a pulpit in permanently supported manways.
Apparatus is also disclosed in the prior application for attachment to the mining machine in order to adapt it to the specific requirements of underground bituminous sands mining, in which a pair of cleaner ploughs are retractably secured to the mining machine at each end thereof, for alternate action in following relationship to the mining machine, and in leading relationship to the advancing apparatus for the mining arch sections, the purpose of which is to remove spalled detrital from the fresh cut which might otherwise impede the forward movement of the mining arch sections.

1(~384~11 Apparatus is also disclosed for indexing the mechansim for advancing the mining arch sections.
It will be appreciated that the method of this prior invention is, in effect a combination of "longwall mining", which is characteristic of coal mining, and "block-caving", which is characteristic of hardrock ; mining, in which the bituminous sand body is continuously block caved behind the advancing undercut of the longwall face, until the bituminous sand body is completely mined out, there being no attempt to support the backs.
In the present application, therefore, a principal objective is also to provide a mine configuration and mining system which adapts readily to the application of the "Fixed Arch Shield" technique of min- - -ing subsurface ore deposits.
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Accordingly, the present invention provides a mine configura-; tion having at least two, but preferably three tiers, or levels. The upper of mining level positioned at or near the base of the ore deposit, ; comprises a tunnel system including peripheral service tunnels bounding , the mining zone, a central access tunnel and a plurality of transverse tunnels extending from the central access tunnel to adjacent service 3 tunnels. The transverse tunnels preferably run parallel one to the other and effectively divide the mining level into a horizontal series of con-tig~ous mining blocks. Each block is mined as will be described hereafter by the "Fixed Arch Shield" method, the ore being conveyed from the face of the block to the transverse tunnels, from where it is transported to vertical shafts, or loading chutes which connect with a lower level of the mine.
In addition to the single mining level, preferably two lower ,~j,.
operating levels are contemplated by the invention. Whether in fact a double or triple mine is developed depends largely on the topography of -the area, size and depth of the ore deposit or type of mineral being mined.
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,` ~38411 All these factors relate to the total cost of operating and consequently have a direct bearing on the final mine configuration.
In the case of two tier mine, the lower, second operating level has the facility for collecting ore passed downwardly from the mining level ~, .
via vertical ore passes, or chutes, and for transporting the ore to the ~ -surface.
More preferably, however, the invention envisages a three-tier mine configuration having a mining level as previously described; a second lower operating level which acts as a collecting and conveying level only, and; a final third, still lower operating level which has the facility for large scale haulage and removal of the ore from the mine. Each level is interconnected by ventilation shafts, and supply shafts which include surface transportation apparatus, and the referred to loading chutes and ~:
ore passes which are located on all levels, at positions which are pre-determined by the layout of the tunnel system, and ore conveyors.
~ Accordingly, a preferred embodiment of the invention will now ; be described, by way of example only, reference being had to the accom-panying drawings which depict a three-tier configuration and the operation thereof.
Figure 1, is a schematic plan of a third lower level showing the layout of haulageways, loading chutes, supply and ventilation shafts;
Figure 2, is a schematic plan of a second, transport and collecting level showing the layout of the major collecting tunnels which serve to convey the broken ore to the loading chutes;
Figure 3, is a schematic plan of a first upper, or mining level, with a central access tunnel and periphery service tunnels related to the supply and ventilation shafts' Figure 4, (Sheet 3) is an enlarged, sectional view along line B - B
of Figure 1 and A - A of Figure 3, showing the three levels, loading chutes, ore passes, oil shale and overburden;
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~384~1 , Figure 5, (Sheet 2) is a schematic plan of a transverse tunnel on the mining level, between production blocks, the central access tunnel and the periphery service tunnels;
`: Figure 6, is a schematic, enlarged plan of a portion of Figure. 5 with production blocks between transverse tunnels extending from central access tunnels; ~.
Figure 7, is a schematic, enlarged plan of same area as Eigure 6 showing first mining cuts advancing from the central access tunnel in six :~
production blocks; . .
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Figure 8, (Sheet 3) is a sectional view along line C - C, of : :
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:~ Figure 7 and shows the oil shale subsiding into the space behind advancing :
. production faces and the subsidence of overburden at the surface filled by waste;
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Figure 9, is a schematic, enlarged plan of same area as Figure i~ 7~ showing subsequent cuts following along the production blocks from the ~-central access tunnel;
: Figure 10, is a sectional view along line D - D, of Figure 9, with the first cut advanced and a second cut following the first;
Figure 11, is a schematic plan of the same area as depicted : in Figures 5, 7, 9, with production faces far advanced and a fifth cut. 20 following earlier cuts; ~
Figure 12, (Sheet 7) is a sectional view along line E - E of Figure ~ :
11 with five successive cuts started and following earlier cuts; ~ ~
Figure 13, is a sectional view along line E - E of Figure 11 ~ .
with twenty cuts started and following previous cuts, oil shale subsiding ~ -:into spaces behind advancing production faces and subsidence of overburden at surface filled with waste;
Figure 14, is a sectional view along line F - F of Figure 11 and at right angles to Figure 13 with some twenty successive cuts started;
Figure 15, is a perspective view of the lining utili~ed in the transverse tunnels, having arches o~ steel or other material.

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Figure 16 is a perspective view showing relationship of the tunnels in three levels and the interconnecting ore passes and loading chutes.
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As previously indicated, this invention is primarily suited for recovery of deep thick mineral deposits such as oil shales, coal and other minerals. Accordingly, property should be purchased to provide sufficient ore reserves within a single rectangular tetragonal block or prism, although irregular blocks can be accommodated.
Because of the diversity of access and topography related to the many occurrences of oil shales in the Western U. S. A., no attempt will be made here to show access. Access to the underground workings could be by shafts, declines, adits or a combination of these.

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Because of the tremendous tonnages involved, either full-sized railway haulage or large sized belt conveyors are indicated for use as the haulage medium.
By means known per se, access is provided to the base of the oil shales.
Mining begins as close to the base of the oil shales as can be ~ attained, and begins at, and advances from, the center of the property and - 20 spreads in all directions from the center.
The most desirable features of this invention are in the control of production, and surface subsidence; the control of the surface indication of subsidence, and in the provision of a safe working environment. The mining machinery in the mining shield is remotely controlled by the operator who is always protected by massive steel structures and remote ` from the actual cutting face and scene of operations, and does not have to come into contact with the ore at any time.
For reference purposes, the mlning shield mentioned through this specification, which as previously indicated, forms the basis of co-pending Canadian Patent Application Serial Number 195,228, is constructed from a '' .

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.: 1~3~411 ~v~ i city of arch sections of the type sh~wn in Figures 15 arranged in side-by-side relation. For clarification purposes the term arch section is used here to ` define a structure having a base or bottom portion and a side wall extending up-wardly at a generally acute angle from the base portion in the form of a half-- arch. There is no permanent interconnection between arch sections, and the : ~ : ., .
complete mining shield is utilized with its open side facing the wall to be mined. The shield is constructed such that each arch section is independently, . forwardly advanceable in the mining direction, the advancement being affected by the mining machine as it traverses the cross-cut.
Subsidence of the oil shale and overburden follows the advance of the mining apparatus, and spreads out from the center of the property. The de-gree of subsidence has no effect on the recovery from the surrounding property.
The underground workings may be expanded, as required, or a completely separate operation commenced exactly at the limits of mining of the first operation.
Recovery of the oil shale, that is of "ore" grade, is invisaged as close to 100%.
Referring to the drawings in general; ~ -Ventilation rises of shafts 36 are installed at the approximate four courners of the property. Exemplary distances between might be that the dimensions indicated, Y, would be five miles.
The "Mining Zone" is centrally located within the rectangle of ventilation shafts 36. The mining zone being that volume of oil shale, within the property, which is under active mining operations. Exemplary dimensions might be that, in a s~uare block 5 miles by 5 miles in extent, the mining zone might be 4 miles by 4 miles with l/2 mile around the sides reserved as a"fender" against the lateral spread of subsidence from the underground work-ings within the mining zone.
At a section substantially bisecting the mining zone, a pair of access and supply shafts 26 would be installed at the outer boundary of the property.

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; Referring specifically to the drawings:
Figure 1 is a plan view of a main haulage level 20 driven be-neath the mining zone. This level is driven in a known manner at the base of the oil shale, or preferably, below the ore zone. The tunnels which - constitute haulage level 20 are driven outwardly from the central access tunnel 35 which also serves as a ventilation tunnel, and are spaced at a distance W, apart. Exemplary dimensions might be that W could be 1 1/4 miles, although the spacing will be such as to give maximum service with minimum cost. At regular intervals governed by the most advantageous loading requirements, loading chutes 22 are installed in the haulage tunnels 20.
The haulage lavel 20, along with all other levels would be connected to the shafts 26 and 36 by suitable manways for emergencies but, for ventilation, the shafts 26 would be "intake", and the shafts 36 "exhaust".
Figure 2, depicts a collecting and gathering level 21, driven above the main haulage level. This second tunnel complex 21 is connected to the main haulage level 20 by the vertical loading chutes 22. The col-lecting and gathering level 21, consists of a network of tunnels 23 con-taining belt or other conveyors, (not shown), converging on the loading chutes 22. Figure 2 is schematic and the actual number of tunnels 23 would be governed by the optimum spacing of the ore passes from the min-ing level above.
The second level 21 is also the exhaust air level. It is connected to the ventilation shafts 36 by tunnels 34, and the draft ad-justed to produce a negative air pressure throughout level 21.
Fi~ure 3, is a schematic plan of the first, or upper mining level 24, which is driven above the transport and gathering level 21.
The mining level 24 consists of a central access tunnel 25 driven between the two access shafts 26. At the limits of the mining block, periphery :' ~0 ,. ~

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; ~38411 service tunnels 30 are driven from the central access tunnel 25. The periphery service tunnels 30 extend around the complete outer periphery of the desired mining zone.
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: Figure 4, is a sectional view, enlarged, along line A - A of Figure 3, and Line B - B of Figures 1 and 3, and shows the three levels , 20, 21, 24, loading chutes 22, ore passes 31, oil shale 40 and overburden 39. When these levels are installed, the mining zone is ready for produc-tion.
From Figure 5, it can be seen that the mining zone is divided into a series of production blocks 29. The width Z of each production - block is dependent on the type of mining shield selected for the develop-ment. An exemplary dimension might be that Z=1000 feet. Transverse tun-nels 27 are driven between production blocks 29 such that they form the outer boundaries of the production blocks 29, and extend from the central ~-~ access tunnel 25 to the periphery service tunnels 30. -; Figure 6 is an enlarged portion of the plan of Figure 5 and -.
shows the transverse tunnels 27 extending from the central access tunnel 25. The production blocks 29, of width z, are enclosed between the tunnels -: 27.
Mining begins at, and advances from, the center of the mining zone, on the mining level, at a point approximately equidistant between ; the access shafts 26. A production block 29, bounded by transverse tun-nels 27 is selected as the location of the first cut.
The "Fixed Arch Shield" technique for mining involves the taking of a horizontal cut from a working face of each production block by mechanical mining machines qperating within the shield. The working face extends between two transverse tunnels 27 and would, therefore, have ;~
length Z. The height of the shield would determine the thickness of cut and an exemplary thickness might be in the region of 10 feet.
According to the present invention two mining shields are set ,''' , :

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up, back-to-back in the central access tunnel 25. The production face 28 is, initially, the side wall of the central access tunnel 25. The machi-nery in the shield excavates a slice of oil shale from the side of the central access tunnel and widens the tunnel 25 over the length Z until subsidence of the roof of tunnel 25 occurs.
The production face 28 advances by repeated slices being excavated from the face 28 as the shield is continually moved up to the face 28, between the transverse tunnels 27 along the base of the produc-tion block 29.
The oil shale material removed from the production face 28 is deposited on chain conveyors, (not shown) in the shield which also act as a track for the cutting machines, (not shown), as in standard long-wall mining procedure. The ore is conveyed horizontally along the face 28 to the transverse tunnels 27, transferred to belt conveyors in the transverse tunnels 27 and conveyed ahead, away from the central tunnel 25.
When a transverse tunnel 27 on the first or mining level 24 crosses a tunnel 23 on the second level 21, a vertical "ore pass" 31 is made connecting the two tunnels 27 and 23 so that the conveyor in the transverse tunnel 27 discharges down the ore pass 31 onto the belt con-veyor in tunnel 23. The ore is then conveyed to the loading chute 22.(ref. Fig. 16) The development of the "Fixed Arch Shield" techni~ue elimin-ates the necessity for supporting the roof or "back" of the workings.
As the production face 28 advances by the operation of the shield, the roof 32, behind the shield subsides into the void 33. Because the width Z of the productionface 28 is large, there is no alternative but for subsidence to occur. The amount of subsidence, from any one cut, is very limited. As an example, if the mining shield is 10 feet high, and ' 10 Z approximately 800 feet, the subsidence would be 800 ft. which is a comparatively minute subsidence and this would produce a limited amount ~ 2 ` ~ : `

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of disturbance immediately above the roof.
When the roof is in contact with the floor, subsidence stops and conditions of equilibrium are re-established.
After subsidence is complete, the central tunnel 25 is re-exca- - - -vated,two more shields are set back-to-back and then advanced outward along the production block to make a second cut, 42.
- Figure 7 is a schematic plan of the same area as Figure 6 show-ing the first mining cuts 41 advancing from the central access tunnel 25 in six production blocks 29 on the minirg level 24.
Figure 8 is a sectional view along line C - C, Figure 7 with the moving production faces 28 producing the first cuts 41. Oil shale is shown subsiding into the space 33 behind the production face 28 with the subsidence 37 extending upward, and spreading out, through the oil shale 40 and the overburden 39.
Figure 9 is a schematic plan of the same area as Figure 7 but with the first cuts 41 being followed by second cuts 42. More production blocks 29 are under active operation and the area of operations is expan-, ded both ahead and laterally from that shown in Figure 7.
Figure 10 is a sectional view along line D - D, Figure 9 with first cut 41 advanced and second cut 42 following. The subsidence 37 in the oil shale 40 and the overburden is more extensive and is being filled in at the surface by waste, 38.
` Fiqure 11 is a schematic plan of the same area as Figures 5, 7, 9, at a more advanced stage of development when the production faces 28 are far advanced with the fifth cut 45 following earlier cuts, 44, 43, 43, (cut 41 already advanced beyond the area). All production blocks are in active development within the selected mining zone.
Fiqure 12 is a sectional view along line E - E of Figure 11 with five successive cuts started and following others, again, oil shale 40 is ~;
shown subsiding into spaces 33 behind the production faces 28, and the : 13 .

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: 1038411 subsidence of overburden 37 at the surface filled with waste 38.
Figures 7 to 12 inclusiver therefore show the progress of the remova~ of tha oil shale, the subsidence of the remaining oil shale and the overburden, and the filling of the subsidence with waste.
Figure 13 is a sectional view along line E - E of Figure 11 at a much more advanced stage when twenty successive cuts have been started and are following the previous cuts. A large part of the oil shale in the mining zone has at this stage been removed from its original position ~ accompanied by the infilling of a large amount of waste.
- 10 Figure 14 is a sectional view along line F - F of Figure 11, which is at right angles to line E - E and to Figure 13 with the same twenty cuts started, the same amount of oil shale removed, same amount of subsidence, and the same amount of infilled waste as shown in Figure 13.
Figure 15 shows the transverse tunnels as being lined with arches constructed preferably of steel, or materials having similar pro-perties. The movement of each successive production face 28 along the length of the production block 29, between the transverse tunnels 27, in the "Fixed Arch Shield" technique, is accompanied by movement and re-arrangement of these arches. The material above the arches is excavated so that there is no build-up of pressure on the arches as the roof or "back" 32 subsides behind the production face 28.
Finally, Figure 16 shows in perspective, the interconnecting of the three levels, 20, 21, and the transverse tunnels 27 of the mining level. The angular relationship of the tunnel system may be chosen to suit a particular mining zone, and therefore is not limited to the con-figuration shown.
The lining of multiple arches, Figures 15 and 16, remains in the transverse tunnels 27, behind the production face 28, as the produc-tion face 28 advanced along the production block 29, but is not occupied, (except for emergency escape). The tunnel 27 continues to provide venti-, ~ " : , . . .

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lation to the produetion face 28.
When a further pair of shields is emplaced, back-to-back, in the central access tunnel 25, after subsidence and preparatory to starting another cut along a production block 29, the transverse tunnels 27 are again available for service of the production face 28 of the new cut.
When the production face 28 is advanced by the action of the shield through the length of the production blocks 29 as far as the ser-vice tunnel 30, the whole mining shield including all machinery, is re-moved and transported via the service tunnel 30 to the central access tun-nel 25 where the shield is reset for a further cut.
The procedure of re-excavating the central access tunnel after subsidence, the setting up of two shields back-to-back and the starting of a further cut is repeated as often an necessary to mine out the thick-ness of oil shale. The number of times this must be repeated is determined by the formula:
thickness of "ore Number of suecessive euts = depth of one eut Control of breeeiation of the oil shales, rate of subsidence, and the rate of production are all provided by regulating the pattern of production bloeks 29, ineluding the width Z, by spacing the sueeessive , . .
euts closer or farther apart, (varying X, Figure 11), and by increasing the number of produetion bloeks 29 under aetive development.
; sreeeiation of the oil shale 40 is desirable for most econom-ical mining beeause it is just as effeetive as conventional blasting tech-niques, but brecciation should be limited to the zone immediately above the mining level 24 so that the whole mining zone does not become too permeable to surface waters.
- The subsidenee due to a single cut is minor. Because of the ` eomparatively large area involved, the subsidence must extend to surface.
However, by the time the subsidence reaches surface it has spread over 30 an area approximately twiee the area of the cut in the produetion bloek 29.

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"' ." '': ' ' , ' ' . ~ .' It will also have only substantially a lf the vertical movement over this greater area. It is therefore manifest, at surface, as a gentle subsi-dence that can only be detected by precise measurements.

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thin layer of tailings 38 is spread, on surface, over the subsiding area. The spreading of tailings is continues as the mining and subsiding continues. ~he apparent effect on surface is that there is a ` continuous spreading of a thin layer. There is no build-up of mountainous piles of tailings. Any increase in the volume of tailings, over the volume of unbroken ore, is absorbed in a slow and scarcely perceptable rise in the surface level. As the depth of tailings increases, compaction of the tailings will occur which will largely nullify the expansion in volume.
As will be appreciated, a continual growth of annual plants, grasses, etc, can be maintained on the surface except for a very small portion where actual filling is in progress. The grasses can reseed them-selves and seeding is constant following filling.
While preferred embodiments of the invention have been shown, it is understood that the invention can be practiced in other ways, and that various modifications and changes can be made in active mining opera-tions which are within the spirit of the invention, and the scope of the .: .
following claims.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of mining subsurface friable mineral ore deposits comprising the steps of:-(a) designating a suitably sized mining zone;
(b) forming a tunnel complex below said mining zone to constitute a main ore haulage level;
(c) forming a tunnel complex spaced above and parallel with said main haulage level but substantially be-low said mining zone, to constitute an ore collect-ing and transport level;
(d) forming shaft means for passage of ore from said transport level to said haulage level;
(e) forming a single mining level at the base of said mining zone, above said transport level, said level having a plurality of service tunnels bounding a designated mining area; a main access tunnel cen-trally bisecting said mining area, and; a plurality of transverse tunnels equidistantly spaced in paral-lel array, and extending from said main access tun-nel to adjacent of said service tunnels, to provide communication therebetween, and to define a multi-plicity of mining blocks;
(f) mining said block means by:
(i) providing a mining shield comprising a multiplicity of independent arch sections arranged in side-by-side relation, each said arch section having a bottom portion and a side wall extending upwardly at a generally acute angle therefrom, said shield being open opposite said side wall to an exposed face of said mining block;
(ii) providing within said shield, mining machin-ery including rotary cutting means adapted for reciprocation across said exposed face;
plough means for removing mined material to said shaft means, and; means associated with said mining machinery adapted to serially ad-vance each said arch section as the mining machine transverses the face, a distance equal to the depth of the mining cut;
(iii) premitting said ore to subside behind said shield as said shield advances through said block; and (iv) continually mining said subsided material by programmed transversing of said shield on said single level until said deposit is exhausted.

2. The method of Claim 1 wherein mining is controlled from a shielded structure positioned within said transverse tunnels at each end of said shield, which structure houses operating per-sonell, and apparatus necessary to effect re-arranging of the arch sections forming said transverse tunnels.

3. The method of Claim 1 including the additional step of providing ventilation ducting to each operating level, and pro-vided a negative air pressure within the tunnel complex of said transport level.

4. The method of Claim 1 including the additional step of controlling the limit, degree, and amount of surface subsidence by dimensional adjustment of said mining blocks, the regulation of the frequency and spacing of mining operations, and by mining in an outward direction from the center of said mining zone.